<!DOCTYPE html>   
<html lang="en">   
<head>   
  <meta charset="UTF-8">   
  <meta name="viewport" content="width=device-width, initial-scale=1.0">   
  <title>let命令-不允许重复声明</title>   
  <script>   
    var x = 3;   
    var x = 4;   
     console.log(x); // 单独运行结果为 4 
      //如果与下面代码一起运行，则无结果，因为下面代码属于编译错误    
     let y = 1;   
     let y = 2;   
     console.log(y); // Identifier 'y' has already been declared      
   </script>   
 </head>   
 <body></body>   
 </html> 